This can be a very difficult task if you are unable to see an actual area that might be torn or leaking. If nothing is visible, you will need to get inside the pool (goggles help!) and scan the side walls carefully, to see if you find any puncture, split, or separation in the liner anywhere. Pay close attention as well to the seams as a leak there will be hard to catch unless you are looking closely for it!
Once you have found the area that is causing the issue, we have several different sizes of pool patches available that can be done under water to fix the issue! Keep in mind that a patch is only temporary, but ultimately a permanent fix would be replacing the liner!